The present invention relates to UV curable organic coating compositions, such as a UV curable organic material, a UV curable organosilicon material, or a mixture thereof, in combination with a photoinitiator in the form of an acylphosphorus compound capable of being activated by light with a wavelength in the range of from about 360 nm to about 410 nm and selected from the class consisting of acylphosphine oxides, acyl phosphonates and acylphosphine sulphides, and up to about 20% by weight of the UV curable coating composition of a UV screen. More particularly, the present invention relates to certain weather and abrasion resistant thermoplastic composites having thicknesses of up to 1 inch, or more which can exhibit improved optical clarity, and which comprise a thermoplastic substrate such as a polycarbonate film or sheet, and a cured polyacrylic or polyacrylic-urethane coating.
Prior to the present invention, as shown by Clark, U.S. Pat. No. 4,027,073, heat curable silicone hard coat formulations were available which could be applied to a variety of thermoplastic substrates, such as a polycarbonate or polyester in the form of film or sheet. Although useful results can be obtained by employing such heat curable coating compositions, an organic solvent is used which is environmentally unattractive. In addition, long process times are required to make a satisfactory abrasion and weather resistant composite of a thermoplastic substrate and hard coat.
Abrasion resistant thermoplastic composites having improved optical properties also can be made by effecting the cure of a solventless radiation curable coating composition which has been applied onto the surface of a polymeric sheet or film. The cure of the applied coating material can be effected while it is in contact with a smooth surface (cold casting) by directing radiant energy through the substrate opposite the surface being coated. The "cold-casting technique" is further illustrated in the copending application of Crouch, Ser. No. 812,619, filed Dec. 23, 1985. Although improved surface characteristics can be achieved with the radiation curable coating compositions of Crouch, the resulting abrasion resistant thermoplastic film can suffer from reduced weathering resistance unless a latent UV screen is used which does not interfere with the cure of the UV curable coating composition while it is on the substrate.
Suitable latent UV screens, are for example, sulfonate esters of hydroxybenzotriazoles, as shown by Olson, U.S. Pat. 4,344,830, which is incorporated herein by reference. These latent UV screens can impart improved weathering resistance over extended periods of time, while simultaneously allowing the cure of the radiation curable coating. Although composites of thermoplastic films having thicknesses up to about 30 mils, or sheets having thicknesses of up to about an inch can be made by treating the films or sheets with a UV curable coating composition and thereafter effecting the cure of the coating composition in the presence of a latent UV screen, experience has shown that latent UV screens often have limited solubility in the coating composition. In addition, these latent UV screens frequently separate from the cured resin after an extended weathering period resulting in a marred, unattractive surface.
It would be desirable therefore to provide solventless UV curable coating compositions which are capable imparting improved weathering and abrasion resistance to a variety of thermoplastic substrates in a relatively short period of time when applied and cured thereon. It also would be desirable to provide composites of thermoplastic films or sheets having a UV cured abrasion resistant polyacrylic or polyacrylic-urethane coating having a thickness of about 4 to 15 microns which can contain an effective amount of a conventional instead of a latent UV absorber, and possess superior optical characteristics.